Changing your fps during game for any reason would be frowned upon, and I and many other hosts would ask you not to return if you were to do that. if you change the weight of bb you dont change anything, its a simply relationship that most fail to understand Ek=0.5mv^2. if you increase bb weight you decrease the velocity but the impact energy remains the same similarly if you decrease bb weight your velocity will increase but the impact energy will be the same, when a game sets an fps limit what they are actually setting is an energy limit for example 420fps on .20g bbs is roughly 1.64j and off the top of my head I believe its 376fps on .25s and it continues to decrease. In any case even if what you are thinking were to be true it would be moronic to use lighter bbs for being a "DMR" as you will loose massive amounts of inertia which translates to a loss of range and accuracy that would massivly offset any gain in fps, if physics actually worked that way, which they dont so dont bother
Furthermore the benefit of say 420fps auto limit vs 450fps limit DMR /w .20g bbs is so little it really doesnt matter, for example my personal gun shooting at 0.97j (340ish fps with .20g bbs) is hitting roughly 250ft where as Thundercactus's m14 is shooting at roughly 415fps, an increase of nearly 100fps is only getting 280ft of range, 30 extra feet for a massive increase in muzzle energy, the difference between the DMR and the standard limit is so small it really wont matter in the end.
also a friendly reminder, no selling outside the classifieds